1. Field of the Invention
This invention relates to an optical head which writes/reads information optically to/from a data medium.
2. Description of Related Art
FIG. 1 is a conventional optical head disclosed, for example, in Japanese Utility Model Application Laid-Open No. 61-68322 (1986). In the figure, a light beam emanated from a semi-conductor laser 1 is deflected by a half mirror 2. In the direction the emanated light is to be deflected, an objective lens 3 is disposed, and at the far side of the objective lens 3, a data medium 4 is disposed. On the data medium 4, a guiding groove 4a is provided along each track where information being is written/read thereto or therefrom.
A 2-divided photodetector 5 having two light-receiving elements is disposed across the half mirror 2 from the objective lens 3. The 2-divided photodetector 5 is connected to a differential amplifier 6. The differential amplifier subtracts the output of the one light-receiving element 5a from the output of the other light-receiving element 5b.
In the optical head as mentioned above, a light beam emanated from the semi-conductor laser 1 is sidewardly deflected at 90.degree. by the half mirror 2. The deflected light is projected on the data medium 4 through the objective lens 3. In the case where the projected light focuses on the data medium 4, a minute light spot of about 1 .mu.m in diameter is formed on the data medium 4. The reflected light from the data medium 4 is projected on the 2-divided photodetector 5 through the objective lens 3 and half mirror 2. The 2-divided photodetector 5 outputs to the differential amplifier 6 an electric signal corresponding to the amount distribution of the light incident upon the two light-receiving elements 5a, 5b. The differential amplifier 6 detects the tracking error of the minute light spot against the data medium 4 on the basis of the amount distribution of the light incident upon the two light-receiving element 5a, 5b.
The tracking servo is carried out by driving the objective lens 3 in the radial direction of the data medium 4 (direction T in the figure) on the basis of the detected tracking error signal. On the other hand, the focusing error of the minute light spot against the data medium 4 is detected by a focus error detecting system (not shown). The focusing servo is carried out by driving the objective lens 3 in the vertical direction to the radial direction of the data medium 4 (direction F in the figure) on the basis of the detected focusing error signal.
Next, explanation will be given on the principle of detecting the tracking error. On the left sides of FIG. 2(a), (b), (c), (d) are shown partial plane views of the data medium 4, and on the right side partial views of optical paths. As shown in FIG. 2(a), (c), in the case where the minute light spot is at the center of the guiding groove 4a or at the center between the guiding grooves 4a on the data medium 4, that is, the writable/readable position of information, the light amounts being incident upon the two light-receiving elements 5a, 5b are equal.
On the other hand, as shown in FIG. 2(b), (d), in the case where the minute light spot is crossing the guiding groove 4a, the light irradiated on the guiding groove 4a is diffracted, and the amount of the light being incident upon the one light-receiving element 5a or 5b (oblique line portion) becomes less than that being incident upon the other light-receiving element 5b or 5a. Accordingly, by detecting the difference between the outputs of the two light-receiving elements 5a, 5b, it can be detected whether the minute light spot is adjusted to the guiding groove 4a or not and to which side the spot deviates. This is the tracking error detecting method to be called the diffracted light method or push-pull method.
But in the conventional optical head as mentioned above, at the time of the tracking servo, when the objective lens 3 is displaced in the tracking direction (direction T in FIG. 1) as shown in FIG. 3 by the broken line, the light being incident upon the 2-divided photodetector 5 moves. Accordingly, there is a problem that the tracking servo cannot be carried out correctly because the detected tracking error signal includes the offset.